A. Picado

Storm surge impact in the hydrodynamics of a tidal lagoon: the case of Ria de Aveiro

ABSTRACT Picado, A., Lopes, C.L., Mendes, R., Vaz, N. and Dias, J.M., 2013. Storm surge impact in the hydrodynamics of a tidal lagoon: the case of Ria de Aveiro. Storm surges are a hazardous phenomenon, since they may flood large coastal areas, causing socio-economical and habitation losses. Thus, the study of their characteristics and effects in coastal regions is crucial to prevent their negative consequences. This work aims at assessing the storm surges impact in the hydrodynamics of a tidal lagoon located in the north-western Portuguese coast (Ria de Aveiro). Storm surge amplitudes of 0.58 m, 0.84 m and 1.17 m for 2, 10 and 100 return periods, respectively, were determined adjusting the annual maximum amplitudes to a Generalized Extreme Value (GEV) distribution. To assess the hydrodynamic changes in the Ria de Aveiro under storm surge conditions, numerical modeling simulations were carried out, considering four scenarios: a single astronomical tidal forcing (reference) and astronomical tide plus 2, 10 and 100 years return period surges. Maximum levels and velocities for the entire lagoon and the tidal prism for the main cross-sections were determined and compared with the reference scenario. Generally, the model results suggest that during storm surge events the maximum levels increase in whole domain, with the largest increase found for the 100 return period storm surge scenario (1.17 m). The most significant changes occur at the main channels head for all scenarios, revealing that these regions are the most vulnerable to marginal flooding. Also, storm surges induce higher velocities and tidal prisms in the lagoon, increasing the marginal risk of erosion, as well as the salinization of the lagoon marginal lands.

Chlorophyll concentration along the northwestern coast of the Iberian Peninsula vs. atmosphere-ocean-land conditions

ABSTRACT Picado, A., Alvarez, I., Vaz, N., and Dias, J.M., 2013. Chlorophyll concentration along the northwestern coast of the Iberian Peninsula vs. atmosphere-ocean-land conditions. The aim of this work is to investigate the relationship between the atmosphere-ocean-land conditions and chlorophyll a (Chl-a) formation along the northwestern coast of the Iberian Peninsula. Spatial and temporal distribution of Chl-a concentration, sea surface temperature, surface winds and rivers discharge were analyzed from 1998 to 2007, using remote sensing data. Generally, the Chl-a concentrations are higher near the coast showing a seasonal variability. In fact, along the coast there are higher Chl-a concentrations during the dry season (April to September) which can be associated to the upwelling favorable conditions (southward winds). These favorable conditions are related to the presence of cold nutrient-rich water which upwells towards the surface layers enhancing the primary production. During winter months, the wind pattern is usually upwelling unfavorable (northward winds); however high Chl-a concentration also occurs near the coast. These high values could be related to the inland nutrients input through rivers discharge and winter upwelling events. Afterwards, correlation coefficients were computed between Chl-a and upwelling index, SST and rivers discharges. Between June to September chlorophyll and upwelling index are positively correlated, increasing southward (0.58 to the south of the study area). During winter, the correlation coefficient between chlorophyll and rivers discharge is 0.75 in the northern region of the study area. Therefore, during summer months the chlorophyll variations could be explained mainly by the frequent upwelling events, whereas during winter months, high chlorophyll concentration near coast depends on rivers discharges.

Tidal dispersion and flushing times in a multiple inlet lagoon

ABSTRACT Lencart e Silva, J. D., Lopes, C. L., Picado, A., Sousa, M. C., Dias, J. M., 2014. Tidal dispersion and flushing time in a multiple inlet lagoon. In: Green, A.N. and Cooper, J.A.G. (eds.), Proceedings 13th International Coastal Symposium (Durban, South Africa), Journal of Coastal Research, Special Issue No. 70, pp. 598–603, ISSN 0749-0208. The Ria Formosa is a tidal multi-inlet shallow-water coastal lagoon located in the south of Portugal, subjected to the dry Mediterranean climate. The tide controls the Rias exchange with the adjacent shelf for most of the hydrological year except for isolated torrential run-off events. Episodes of low hypoxia reported in the literature may be related to the lagoons flushing time, affecting its shellfish production valued at 20 – 50 million €y−1. Over the past decades several observational and modelling studies presented values for the capacity of the tide to renovate the water inside the Ria. However, these studies lack either the spatial resolution to yield results unaffected by numerical diffusion or analyze a very limited part of the lagoons territory. In this work, we use a very-high resolution hydrodynamic model to assess the flushing time exclusively due to tidal forcing inside the Ria Formosa. A bi-dimensional implementation of the finite-volume/finite-difference Eulerian Lagrangian hydrodynamic and transport model (ELCIRC) was used, allowing for the local refinement of the computational domain, which best suits the lagoons complex morphology. The present model configuration was validated for tidal propagation with sea surface elevation collected in 1979/80 at 11 lagoon stations. The validation results show a good agreement between predicted and observed elevations, with root mean square errors lower than 20 cm and skill values higher than 0.98. A set of experiments were carried out by releasing a conservative tracer at different stages of the tide at discrete points of the Ria, where possible environmental hazard hot-spots are located and the flushing e-folding time calculated from the tracers dilution. The results are discussed taking into account the propagation of the tide in this multi-inlet, meandering topography. Evidence is presented of topographic trapping due to the complex spatial distribution of the phase lags of the semi-diurnal tidal constituents. This evidence explains the significant increase of the flushing time from the inlets to the head of the channels, thus justifying the use of detailed spatial resolution when modelling such a complex system.

Biological response of a coastal plain estuary to torrential episodes: a modelling study

ABSTRACT Longo, R.S., Picado, A., Vaz, N. and Dias, J.M., 2016. Biological response of a coastal plain estuary to torrential episodes: a modelling study. In: Vila-Concejo, A.; Bruce, E.; Kennedy, D.M., and McCarroll, R.J. (eds.), Proceedings of the 14th International Coastal Symposium (Sydney, Australia). Journal of Coastal Research, Special Issue, No. 75, pp. 78–82. Coconut Creek (Florida), ISSN 0749-0208. Estuaries are highly dynamic systems with an important impact on biogeochemical cycles and primary production, which may be affected and modified in a climate change context, namely due to extreme rainfall events. This study aims to research chlorophyll-a (Chl-a) and nutrients dynamics in the Tagus estuary under extreme freshwater discharge in a climate change context, using a 2D biophysical model. Three scenarios were set changing the inputs from the main tributaries – Tagus and Sorraia rivers. First, a scenario with one day of extreme discharge for both rivers was considered. Next, and in order to understand the importance of each river, two more scenarios were set considering the extreme discharges separately. Results show that Chl-a concentrations follow the same trend as the imposed discharges, however with a delay of one day. The results also reveal that the biogeochemical characteristics of the Tagus estuary are mainly influenced by the Tagus River inflow. Moreover, in the scenario where the extreme discharges are imposed for both rivers, Chl-a levels increase in the entire estuary and consequently a decrease in nitrate concentrations is observed. Otherwise, phosphate concentrations slightly increase. This suggests primary producers inside the estuary preferentially consumes nitrate, at a higher rate than it is being loaded.

Study of suspended sediment dynamics in a temperate coastal lagoon: Ria de Aveiro (Portugal)

ABSTRACT Plecha, S., Picado, A., Chambel-Leitão, P., Dias, J.M., Vaz, N., 2014. Study of suspended sediment dynamics in a temperate coastal lagoon: Ria de Aveiro (Portugal). In: Green, A.N. and Cooper, J.A.G. (eds.), Proceedings 13th International Coastal Symposium (Durban, South Africa), Journal of Coastal Research, Special Issue No. 70, pp. 604–609, ISSN 0749-0208. Suspended sediment concentrations are simulated at Ria de Aveiro, a lagoon located in a temperate climate area in the northwest of Portugal. The fine-grained suspended sediment concentration is analyzed using the numerical model MOHID (www.mohid.com) and spatial maps of instantaneous and maximum concentration and also temporal variability at specified locations are analyzed in order to characterize the influence of the tide and sea level conditions in the suspended sediment concentrations within the lagoon. The highest suspended sediment concentrations were found in upstream areas during ebb conditions due to the rivers proximity, while the minimum concentrations were observed near the inlet due to the presence of marine water poor in suspended sediments. When a 0.42 m sea-level rise (an estimate of conditions in 2100) is modelled, a decrease in suspended sediment concentration is observed for the overall study area, as result of an increase in the tidal prism.